1. Field of the Invention
This invention relates generally to measuring and testing. More particularly, the invention relates to the tensile and compression strength testing of a rock formation -- esp., an underground rock formation -- in situ.
2. Prior Art
Caverns in rock formations -- e.g., solution caverns in salt domes or bedded salt -- are being used increasingly for the storage of petroleum products (e.g., natural gas) and other important fluids under pressure. As a consequence, the surrounding rock formations are loaded by both tensile and compressive stresses. Upon the pressurization of the caverns, fissures in the surrounding rock formations sometimes occur, with resulting losses of valuable fluid products.
An urgent need therefore exists for a non-destructive method which would afford the effective, rapid, and economical measurement of the tensile and compression strengths of rock formations -- esp. underground rock formations -- thereby providing a positive determination of the suitability of such rock formations for the storage of fluids under pressure.
Accordingly, a number of attempts have been made by prior inventors to provide such a method. As an example, Hardy and Jayaraman have proposed a method in the Society of Petroleum Engineers Journal, June, 1972, at p. 246. Howsoever efficacious, this method is found wanting, in that samples of rock must be removed from the formation in question and taken to the laboratory for testing, thereby consuming valuable time. A number of other proposed methods of the prior art also suffer from this disadvantage. Additional methods and devices of note are found in the following U.S. Pat. Nos. 3,796,091 (Serata); 3,446,062 (Goodman); 3,364,737 (Comes); and 2,957,341 (Menard). However, none provides the desired combination of non-destructivity, rapidity, economy, and efficiency in the measurement of tensile and compression strengths of rock (as distinguished from the measurement of stress, modulus, etc.).